The present invention relates to a bidirectionally pumped optical amplifier and to a method for bidirectionally pumping an optical amplifier. The present invention also relates to a method for stabilizing the optical emission of a laser module including a selective reflector injected by an external radiation.
FIG. 1 shows a configuration of a known bidirectionally pumped optical fibre amplifier 100, comprising an amplifying doped optical fibre section 101, for example an erbium doped amplifying fibre, pump lasers 102, 103, WDM couplers 104, 105, optical isolators 106, 107 for light signals, input and output terminals 108, 109. An optical signal is launched in the amplifier 100 through the input terminal 108, travels along the doped fibre section 101 to be amplified therein and exits through the output terminal 109. Suitable energy for amplification is provided by pump lasers 102, 103, which couple pump light to the doped fibre 101 through WDM couplers 104, 105. In particular, pump light from laser 102 is launched co-directionally in the doped fibre 101, that is in the same direction of the optical signal, whereas pump light from laser 103 is launched counter-directionally, that is, in the opposite direction with respect to the optical signal. For an erbium doped fibre amplifier, pump lasers 102, 103 may emit light whose wavelength is comprised, for example, in a pumping band centered around 980 nm or 1480 nm.
Herein and in the following of the description, the expressions xe2x80x9cco-directionallyxe2x80x9d, xe2x80x9ccounter-directionallyxe2x80x9d, xe2x80x9cco-propagatingxe2x80x9d, xe2x80x9ccounter-propagatingxe2x80x9d will be always referred to the propagation direction of the optical signal.
The configuration shown in FIG. 1 has a problem in that the residual pump light frown each pump laser, not fully absorbed by the amplifying fibre, is injected into the opposite pump laser, which can result in optical instabilities in the emission of the latter and in fluctuations in amplification of the optical signal.
It is known that such instability can be avoided by placing an isolator on the optical path of each of the pumps.
In U.S. Pat. No. 5,640,268 to Alcatel N.V. a further solution is addressed to this problem. According to ""268 patent, each pump injection fibre includes a photorefractive pump filter constituting part of the resonant cavity of the associated pump laser, the two pump filters being mutually different to give rise to an offset between the two pumping bands. The two pumping bands are preferably offset by several nanometers. The pump filters are photorefractive gratings having a determined pitch and thus a determined central wavelength for reflection: according to the ""268 patent, the use of such a grating makes it possible simultaneously to reduce the width of the pumping band and to position said band more accurately within the spectrum.
Applicant has experimentally verified that in a configuration according to the ""268 patent, if the wavelength emitted by the pump lasers is within the 980 nm pumping band, the offset between the two pumping bands should be greater than 15 nm in order to avoid instabilities due to residual pump injection. Since for an erbium doped fibre amplifier, the pumping band centered around 980 nm is only 10-15 nm wide, an offset between the two photorefractive filters of 15 nm or more would lead one of the pump wavelenghts to be nearly out of the pumping band of erbium, considerably reducing the bidirectional pumping efficiency.
Applicant has found that it is possible to avoid instabilities in an optical amplifier bidirectionally pumped by 980 pump lasers, each being stabilized by a respective grating, by using one isolator on the output optical path of one of the two pump lasers and by keeping the output power of the second laser sufficiently higher than the residual injected pump. This allows to use only one isolator for the pump light in the bidirectionally pumped amplifier. A simple feedback apparatus may control the power of the residual pump and the output power of the pump laser with no isolator, in order to maintain the correct operative range.
In a first aspect, the invention relates to a bidirectionally pumped optical amplifier comprising an active fibre having two ends, a first WDM coupler and a second WDM coupler coupled to said ends, a first pump branch coupled to the first WDM coupler comprising a first laser and a first selective reflector, for introducing pump radiation into the active fibre in a first direction, a second pump branch coupled to the second WDM coupler comprising a second laser and a second selective reflector, for introducing pump radiation into the active fibre in a second direction, opposite to the first direction, the second laser being adapted for emitting an output power PF, the amplifier being adapted for coupling a pump residual, having a power Pinj, from the active fibre into the second laser, characterized in that the ratio PF/Pinj is higher than two and the first pump branch further comprises an optical isolator for the pump radiation.
Preferably PF/Pinj should be higher than five, more preferably higher than ten, even more preferably higher than 20.
In preferred embodiments, being xcex1 the center wavelength of the first selective reflector and xcex2 the center wavelength of the second selective reflector, |xcex2-xcex1| is lower than or equal to 15 nm. Preferably, |xcex2-xcex1| should be higher than 5 nm, more preferably higher than 7 nm.
Preferably, being g(xcex) the gain curve of the second laser, the difference g(xcex2)-g(xcex1) should be higher than 0 cmxe2x88x921, more preferably higher than 3 cmxe2x88x921, even more preferably higher than 5 cmxe2x88x921.
Advantageously, xcex1 and xcex2 are comprised in a range between 971 nm and 986 nm.
Typically, the active fibre is an erbium doped fibre. Typically, the amplifier further comprises at least one optical isolator for an optical signal. Preferably, the first and second lasers are semiconductor lasers.
In a preferred embodiment, an optical amplifier comprises a pre-amplifying section, including at least one active fibre and at least one pump laser, and a booster section, including a bidirectionally pumped optical amplifier according to the first aspect of the invention.
Advantageously, an optical amplifier comprising a bidirectionally pumped optical amplifier according to the first aspect of the invention may be included along the optical transmission path of an optical transmission system comprising at least one transmitter and at least one receiver, coupled to the optical transmission path. Preferably, the optical transmission system is a WDM system.
In a second aspect, the invention relates to a method for pumping an optical amplifier comprising an active fibre, a first pump laser and a second pump laser, the method comprising:
coupling a first wavelength-stabilized pump radiation from the first pump laser to the active fibre in a first direction;
coupling a second wavelength-stabilized pump radiation from the second pump laser to the active fibre in a second direction, opposite to said first direction, the second pump radiation having a power PF;
coupling a pump residual in the first direction from the active fibre towards the second pump laser, the first pump residual having a power Pinj;
maintaining the ratio PF/Pinj higher than two, and
blocking a pump residual in the second direction from the active fibre before the first pump laser.
Preferably, the ratio PF/Pinj should be maintained higher than five, more preferably higher than ten, even more preferably higher than twenty.
In preferred embodiments, being xcex1 the wavelength of the first wavelength-stabilized pump radiation and xcex2 the wavelength of the second wavelength-stabilized pump radiation |xcex2-xcex1| is lower than or equal to 15 nm. Preferably, |xcex2-xcex1| should be higher than 5 nm, more preferably higher than 7 nm.
Preferably, being g(xcex) the gain curve of the second laser, the difference g(xcex2)-g(xcex1) should be higher than 0 cmxe2x88x921, more preferably higher than 3 cmxe2x88x921, even more preferably higher than 5 cmxe2x88x921.
Advantageously, xcex1 and xcex2 are comprised in a range between 971 nm, and 986 nm.
In a third aspect, the invention relates to a method for stabilizing the optical emission of a laser module injected by an external radiation having a wavelength xcex1, the laser module including a laser and a selective reflector, the laser having a gain curve g(xcex), the method comprising:
choosing a center wavelength xcex2 of the selective reflector so that g(xcex2)-g(xcex1) greater than 0 cmxe2x88x921, and
maintaining a ratio between an output power PF of the laser module and a power Pinj of the external radiation higher than two.
In preferred embodiments, |xcex2-xcex1| is lower or equal to 15 nm.
Preferably, the ratio PF/Pinj should be maintained higher than five, more preferably higher than ten, even more preferably higher than twenty.
Preferably, the center wavelength xcex2 should be chosen so that g(xcex2)-g(xcex1) greater than 3 cmxe2x88x921, more preferably so that g(xcex2)-g(xcex1) greater than 5 cmxe2x88x921.
Advantageously xcex1 and xcex2 are comprised in a range between 971 nm and 986 nm.